membrane exclusion size

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Osmosis displays itself in most biological systems that are cellular. The cell wall is a semi-permeable membrane. So is the cell nuclear wall which keeps DNA molecules inside the nucleus while allowing the transfer RNA molecules to pass through in and out of the nucleus. Nutrients,oxygen, water, and waste gases can pass in or out of the cell through the semi-permeable cell membrane. Cells in which water passes out of the cell faster than can get in are said to shrink and undergo crenation. This happens when cells are placed in an extracellular fluid of more than .9% salt solution. The extracellular solution is concentrated enough to allow water molecules to osmotically pass to the outside of the cell faster than water can pass into the cell. As a result the cells dehydrate and shrink. This is what happens if a person dying of thirst on a raft in the middle of the ocean decides to drink sea water. The sea water increases the salinity of the extracellular fluids, and a condition arises where more water osmotically leaves the cells than can get out. The cells dehydrate, and ironically, the person suffers from cellular dehydration which becomes more severe as the person takes in more sea water. Such an extracellular solution is called a hypertonic solution.

On the other hand if cells are placed in a salt solution of less than .9% then the water on the outside of the cell in the extracellular fluid can pass into the cell faster than water can get out. The result is that the cells swell with the excess water and eventually burst open. When red blood cells do this it is referred to as hemolysis. The extracellular fluid itself is said to be hypotonic. A condition known as water intoxication results in too much water being ingested where the Sodium/Potassium ion pump can't control the water intake into the cells and the cells rupture due to this intoxication.

Pysiological saline (salt) solution is 0.9% and results in an equilibrium between osmosis into the cell and out of the cell. This kind of solution is the extracellular fluid that our cells respond most positively to. The solution is said to be isotonic. This intricate balance in osmosis in and out of the cells needs to be maintained at all times. The controller of this process is the kidneys themselves which filters out waste products and recycles needed nutrients while maintaining this isotonic condition in the extracellular fluids.